Crystal structures and electrical conducting/magnetic properties in 1: 1 FeCl₄ and FeBr₄ salts of dimethylthio- and ethylenedithio-(1,3-dithiolylidene)thioxotetrathiafulvalene radical cations

The 1:1 radical cation salts of new donor molecules, dimethylthio- (1) and ethylenedithio-(1,3-dithiolylidene)thioxotetrathiafulvalenes (2) with a magnetic FeCl₄ or FeBr₄ counteranion (1·FeBr₄, 2·FeCl₄ and 2·FeBr₄) were prepared, and their electrical conducting and magnetic properties were investigated. The room-temperature electrical conductivities of 1·FeBr₄ (compressed pellet), 2·FeCl₄ (single crystal) and 2·FeBr₄ (single crystal) were < 10^-5, 1.8 × 10^-4 and 7.0 × 10^-2 S cm^-1, respectively. The temperature dependence of paramagnetic susceptibility obeyed the Curie-Weiss law in all cases, and the Curie constant (C) and Weiss temperature (θ) were as follows: C=4.61 emu K mol^-1 and θ=-5.4 K for 1·FeBr₄; C=4.53 emu K mol^-1 and θ=-13.4 K for 2·FeCl₄; C=4.55 emu K mol^-1 and θ=-31.2 K for 2·FeBr₄. The remarkably different θ values between the three salts, considered together with their crystal structures, suggest significant interaction between Fe(III) spins by aid of π spins on 1 or 2 molecules.